A conventional seat apparatus for a vehicle has a reclining mechanism for adjusting a tilt angle of a seat back to a desired tilt angle in order to secure a comfortable sitting position and a safe driving posture for an occupant. Moreover, some of the seat apparatuses for vehicles have a two-mode folding mechanism for folding a seat back so as to be positioned in a reclined position and a forward folded position. The seat back is folded forward to be approximately parallel to a seat cushion by means of the two-mode folding mechanism so that a back face of the seat back can be used as a table and that a loading space for luggage can be created.
In particular, for example, a conventionally known seat apparatus for a vehicle described in JP8214981A includes a base plate (a lower arm) supported on a seat cushion, an upper arm supported on a seat back, and a lower arm supported on the base plate via a first hinge shaft (i.e., a center shaft) and rotatably supported on the upper arm via a second hinge shaft each serving as a component of a lock mechanism. The lock mechanism is configured as follows. In the lock mechanism, the lower arm is rotated via the first hinge shaft while the upper arm is rotated via the second hinge shaft. The upper arm is tilted forward relative to the lower arm so as to cover the lower arm. While a reclining mode is set, the upper arm and the lower arm are simultaneously rotated with each other. Only the upper arm is rotated in order to fold the seat back forward.
Further details of the lock mechanism will be explained as follows. The lower arm and the upper arm are connected to each other via an upper base plate. Teeth for restraining and locking a forward tilting movement of the upper arm are formed at an upper position on an outer periphery of the upper base plate. Teeth for keeping and locking the forward tilting movement of the upper arm are formed at a forward position in an intermediate part on the outer periphery of the upper base plate. A lower side of the upper base plate is fixed to the lower arm. A tooth member is attached to the upper arm to be located at an upper portion of the upper base plate. The tooth member includes teeth at one end engaging with and disengaging from the teeth of the upper base plate. The other end of the tooth member is located at an opposite side of the teeth so as to be rotatably supported on a shaft arranged at the upper arm. The tooth member is rotated around the shaft. Furthermore, a lever for folding the seat back forward and a cam mechanism connected to the lever are disposed at an upper end portion of the tooth member. The lever and the cam mechanism are rotated around a shaft arranged at the upper arm. An approximately triangular shape holder plate having the second hinge shaft and two of the above-mentioned shafts as apexes is disposed in an interior side (in a central side of the seat) of the upper base plate, the tooth member, and the cam mechanism so as to engage with and overlap one another. When the seat back is being folded forward, the tooth member is rotated via the cam mechanism by operating the lever, thereby disengaging the teeth of the tooth member from the teeth of the upper base plate. Then, in accordance with a forward folding movement of the seat back, the lever, the cam mechanism, the tooth member, and the holder plate are rotated along with the upper arm around the second hinge shaft.
When the seat back is used as the table, a position of the seat back is switched from a seated position to a forward folded position (a table position) and vice versa. In this case, the seat back may not be tilted smoothly in the process of switching the position of the seat back from the seated position to the forward folded position (the table position) and vice versa.
Moreover, the reclining mechanism of the seat apparatus is easily assembled to the seat because the reclining mechanism is generally unitized. However, the lock mechanism for folding the seat back forward is not unitized as disclosed in the above-mentioned patent document. During assembling of the seat, various components of the lock mechanism are separately assembled to the seat. In the case where the lock mechanism is not unitized, some of the components may be forgotten to be attached to the seat and a wide space is required in order to arrange the components as well as costs increase because the number of fasteners used for fastening the lock mechanism to the seat increases. Additionally, since the components of the lock mechanism for folding the seat forward are not unitized, the lock mechanism is not strong enough to resist against impact load in a width direction of the seat. Accordingly, reinforcing components against the impact load are required.
A need thus exists for a seat apparatus for a vehicle, which is not susceptible to the drawback mentioned above.